In converters of this kind for high voltages it is previously known to build the valve assembly or assemblies in the form of one or more vertical columns standing on a base, with the valves of a valve assembly resting on top of each other. Especially at high voltages and in the case of converters having two or more bridges which are series-connected on their d.c. sides, each valve assembly will be very tall.
Each valve must have vertical supporting insulators between its two ends, which provide the necessary electrical insulation between the ends of the valve and which may take up forces from valves positioned above it. When the valve assemblies are tall, these forces are great and the supporting insulators must then be heavily dimensioned and are expensive and bulky.
In certain cases, for example if the valve assembly has to be dimensioned to withstand seismic forces or when positioning the converter on a movable base, the dimensioning forces on the supporting insulators become so great that it is difficult or impossible to construct sufficiently strong valve assembles with the prior art technique. Such conditions can arise, for example, in the case of marine installations, by which we mean not only ship installations, but also, for example, installations on oil or gas platforms, where the movements caused by waves, especially during transportation of the platform, may be considerable.
Valve assemblies built up in the manner described above often acquire a mechanical natural frequency which may coincide with the frequency of the movements of the base that may occur, for example during earthquakes. This circumstance may give rise to resonance phenomena and a sharp increase of the mechanical stresses on a valve assembly.
In addition to stresses caused by movements of the base, a valve assembly may be subjected to strong mechanical stresses, for example, when carrying out testing and service.
In addition to the supporting insulators included in the individual valves, such insulators are often positioned at the very bottom of a valve assembly, the whole assembly resting on these insulators which provide electrical insulation between the lower end of the assembly and ground. These insulators are subjected to very great forces, which further accentuates the problems mentioned above.
A valve assembly of the known type described above is usually constructed from a number of sections mounted on top of each other, each section having a metal frame at its upper and lower ends, between which supporting insulators are arranged. These frames complicate the valve assembly and render it more expensive. Furthermore, for reasons of electrical strength, these frames make it necessary to make the assembly taller than if no frames had been used, which involves considerable disadvantages from the points of view of economy and strength.
The present invention aims to provide a converter in which the valve assembly or assemblies may be constructed in a simpler and more economically advantageous manner than before and in which even tall assemblies may be caused to withstand forces caused by possible movements of a base on which the converter is located. Another aim of the invention is to provide a converter having a valve assembly of reduced height compared with previously known designs.